There are a number of possible reasons for this colour change in your Ficus Ginseng.
Let’s start with something which might not be a problem: your Ficus Ginseng tree is going dormant just like the big ones in the woods because of the time of year.
Not enough light: insufficient exposure to high-quality light (indirect light is what is recommended) will cause this problem.
Root problem: if the roots are rotting, the Ficus Ginseng needs to be transplanted, or it could be that there is no drainage hole at the bottom of the pot. You need to remove the tree from its pot and have a look at the roots. If they are mushy and black, or soaking wet, then you have root rot and the tree must be transplanted if it is to be saved.
Location: not many people know that only tropical and some semi-tropical species of bonsai trees can be grown indoors. There are many cases in which bonsai trees sold commercially as indoor bonsais are not really indoor bonsais. They are meant to grow outdoors, and if you don’t change the location your tree will slowly die.
Watering: have you considered the possibility that you are not watering it enough? This is a common cause of this problem. Your Ficus Ginseng needs to be kept moist at all times.
Insect attack or fungal attack: check the underside of the leaves for the presence of very small insects. If you find little dots on the leaves, it is possible that your bonsai tree is being attacked by fungus.
If you own an indoor bonsai tree, be sure that no air conditioner is blowing directly onto it.
If you own an indoor Ficus Ginseng tree, it may lose leaves after being moved to a new location.